Concept on positioning of Columns
Question 1:
Comment on the layout of the framing plan concerning earthquake resistance in the two horizontal directions X or Y (dots are columns, lines depict beams). Look into aspects of symmetry, redundancy, bi-directional stiffness, and ductility.
Answer:
1. Investigation of the framing plan concerning Symmetry in X-direction:
If we split the plan vertically in half, it will look mirrored. So we conclude the given plan is symmetric in the X-direction.
2. Investigation of the framing plan concerning Symmetry in the Y-direction:
If we split the plan horizontally in half, it will look mirrored. So we conclude the given plan is symmetric in Y-direction.
3. Investigation of the framing plan concerning redundancy in X-direction:
The columns in the x-direction are comparatively more and evenly spaced. So the structure is more redundant in the x-direction. Except for the top and bottom columns, rows are having quite longer spans.
4. Investigation of the framing plan concerning redundancy in the Y-direction:
The columns in the y-direction are comparatively lesser but evenly spaced. So the structure is redundant but lesser than that in the x-direction.
5. Investigation concerning Bi-directional stiffness:
The columns in the horizontal directions are connected with straight to straight beams. This means that any lateral load (Seismic force in X-direction) passing from the left end of the building will have its path straight till it reaches the right end of the building. So no or less possibility of force diversion taking place.
The columns in the vertical directions are not connected with straight to straight beams. So a lateral load passing from the south side of the building will get diverted to east & west directions to reach the next bay. There will be major force diversion taking place with Seismic force in Y-direction. The framing plan is weak concerning bi-directional stiffness in Y-direction.
6. Investigation of the framing plan concerning ductility:
Prima facie, the aspect ratio of the plan hints at more columns in direction compared to the y-direction. It will result in higher stiffness of the building in the x-direction, which would reduce ductility on the x-axis.
Moreover, the x-force will easily pass from one bay to the next due to beam alignment. Whereas, the y-force will struggle to pass through.
The north-south beams are arranged in a zig-zag which leads to discontinuous bracing.
We can conclude that the given framing plan is more ductile in the y-direction than that in the x-direction.
Question 2:
What are the possible sources of irregularity in the building shown here? Explain how, as an engineer, each of these irregularities should be dealt with.
Answer:
1-Vertical Irregularity:
There is a change in the horizontal dimension of resistance to lateral forces due to the open floor at mid-height as well as the vertical geometry of the building, which means that the length of the highest floors varies along the building unit.
This causes torsional effects in the building that need to be separated by providing shear walls or separated moments.
The flexibility provided as a result of deviations must be compensated by providing ductility.
The building plan area is large in the bottom stories, and the floor plan areas are getting reduced with an increase in height. These building offsets have to be cross-checked concerning IS-1893-2016 Table 6 serial number 3. In this picture, we can see the bottom 5 storey has a larger area when compared to the upper levels. This is called vertical irregularity.
2-Soft Storey:
As we can see in the picture above, this building on the 6th floor has a soft floor because it has an uncovered opening.
There is no partition wall or any casting or any other shear wall or any other type of supporting elements etc., nothing to support the upper blocks of the building during an earthquake and this will also cause the
plastic hinges on the columns.
In the building there is uncovered opening space on the 6th floor there are no partition walls so, it’s a soft storey, and there is no bracing, no mass, and no occupancies. This will lead to plastic hinge formation of the columns. So, concerning IS-1893-2016 figure 4A. This has to be checked and sufficiently designed to avoid the plastic hinge.
3-Sloppy Base:
- If we observe correctly, we can see that there is a sloping terrain on the right side of the picture and this building is built on this sloping area.
- This means there is a risk that this building could collapse during an earthquake.
- And this will lead to fluctuations in stiffness, as we can correctly conclude.
On the right side of the building, there is an embankment. It means the building is constructed over a sloping ground and this will lead to a significant variation in the stiffness (concerning IS-1893-2016 Figure 5E). So, a few levels may be subjected to the soft Storey mechanism.
Question 3:
A 4-storey hotel building has an open ground floor for the restaurant. storey 2 to 4 has one row of rooms along each long side in the plan, separated by a corridor. The two short sides of the perimeter are fully in-filled in all storeys, except for certain openings at the ends of the corridor at storeys 2 to 4 and along the right-hand side of the ground floor. There is a staircase near the upper left-hand corner, with straight flights between landings at floor levels and in-between floors. Cross-section dimensions are written next to the member no. in meters.
Comment on the structural design features and the layout of infills, which are important for earthquake resistance and seismic performance. How do they relate to the almost full collapse of this building (the extreme left-hand bay with the staircase survived, as well as one long-side façade in the rear end, and the frame along the right-hand side in the plan)?
Answer:
1. Open ground floor:
The ground floor of the building is open. This denotes a soft storey mechanism acting in the building, which can be a soft floor (with long columns), due to the ground floor excitation, the columns would not be able to withstand the earthquake and collapse.
This is called the soft palate mechanism.
This can be avoided by using the strong post and weak beam mechanisms.
2. Two short sides of the perimeter filled:
The mass was not distributed on all sides of the building on all floors because it is given that both sides of the perimeter of the building are filled (can be soil), which causes an irregular distribution of the mass called mass Irregularity.
This can be avoided by providing shear walls in the directions of the two short sides of the perimeter, which can resist the lateral load (soil caused by the fill) by providing rigidity to the building frame and helping to resist the effects of earthquakes.
It also adjusts the mass irregularity for the plan.
3. Extreme left bay Stairs filled with concrete:
The extreme left bay has a Staircase (concrete), thanks to which the mass was concentrated in the space of the staircase, which creates torsional effects based on the concentration of mass in the Staircase.
To avoid this type of failure, the stairs should be located in the center of the building, which will cause the torsional effect to be zero by disconnecting the moment. It also has the consequences of irregularity in the building's floor plan.
4. The columns are not on the same row:
It is observed that the columns in the columns of the extreme left center line have a deviation in the construction line at the staircase, which is again the reason for the irregularity in the floor plan with torsional flexibility.
To prevent this, the eccentricity should be maintained so that the deviation is within the limits of beam size and column size.
Also, the sizes of beams and columns in the plan should be reworked so that the line of columns does not deviate even in the area of the staircase.
5. Staircase portion at Left top corner:
Usually, staircase and lift portions are stiffer than other parts because of concentrated concrete elements. So the staircase portion contains more stiffness than the provided building plan.
6. No framing connection along Y-direction:
We could see no structural beams provided along the vertical direction between columns. This makes the building more vulnerable to the North-South seismic force.
7. Beams connected along column minor axes:
The columns are having a longer dimension in Y-direction. This means they are stiffer along the y-direction. But the beams are running only along x-direction which would attract much lesser lateral forces and hardly transfers it.
8. Variation in column sizes:
The internal columns are having higher sizes compared to exterior columns (This is common because of higher load demands). But this will lead to a significant difference in the lateral drift of columns. So the concrete floor might have suffered heavy out-of-plane shear which would lead to collapse.